In recent years, in the field of automobiles for example, as weight reduction and output enhancement are more highly required, a high stress design is directed also in a valve spring and a suspension spring used for an engine, a suspension and the like, and a clutch spring and the like. Therefore, for these springs, ones which are excellent in fatigue resistance property and setting resistance property are strongly required to cope with increase in load stress. In particular, with respect to a valve spring, the requirement for increasing fatigue strength is very strong, and even SWOSC-V (JIS G 3566), which is said to be high in fatigue strength among conventional steel, is hard to cope with.
In spring steel material wherein high fatigue strength is required, it is necessary to reduce hard nonmetallic inclusions present in the steel as much as possible. From such a viewpoint, with respect to the steel used for the usage described above, it is common that high cleanliness steel wherein presence of nonmetallic inclusions described above is decreased as much as possible is used. Because the risk of wire breakage and fatigue breakage due to nonmetallic inclusions increases as high strengthening of material progresses, the requirement for reduction and miniaturization of the nonmetallic inclusions described above which becomes its main cause has become greatly severe.
From the viewpoint of realizing reduction and miniaturization of hard nonmetallic inclusions in steel, a variety of technologies have been proposed so far. For example in the Patent Document 1, it is disclosed that, in valve spring steel, if controlled to CaO—Al2O3—SiO2 three-component based inclusions whose melting point is lower than approximately 1,400-1,500 DEG C., they do not become the start point of fatigue failure and fatigue properties improve.
Also, in the Patent Document 1, high cleanliness steel, whose cold workability and fatigue property are improved by stipulating the average composition of nonmetallic inclusions so that the nonmetallic inclusions are well stretched in hot rolling and are crushed and finely dispersed in a cold rolling or wire drawing process, is realized. Further, in the Patent Document 2 also, the composition of nonmetallic inclusions which are easily stretched and refined in hot rolling, cold rolling and drawing is stipulated.
On the other hand, in the Patent Document 3, a technology is disclosed wherein the melting point of an oxide-based inclusion is stipulated as 1,500 DEG C. or below to make it an inclusion having a low-melting composition easily stretched in hot and cold rolling.
Further, in the Patent Document 4, as an inclusion having a low-melting composition easily stretched in hot rolling and cold rolling, Al2O3—SiO2—MnO based one containing MgO and/or CaO is stipulated.
Non-patent Document 1: “182nd and 183rd Nishiyama Memorial Technical Lecture”, edited by The Iron and Steel Institute of Japan, P. 131-134. Patent Document 1: Japanese Unexamined Patent Application Publication No. S62-99436
    Patent Document 2: Japanese Unexamined Patent Application Publication No. S62-99437    Patent Document 3: Japanese Unexamined Patent Application Publication No. H5-320827    Patent Document 4: Japanese Unexamined Patent Application Publication No. S63-140068